Traynor YBA-1 Odd Output

[mhuss]

I have a YBA-1 on the bench (a later '74-ish model), with a brand new Hammond OT (the one they make as a YBA-1 replacement). The amp has an oddly distorted output (see attached). The signal on both input grids looks clean, and I tried replacing the output tubes with no change. I carefully checked the socket solder joints, everything looks OK. Any suggestions?

At low levels, it looks clipped on the positive side, at the signal gets hotter, the 'divot' in the top of the waveform gets deeper.

The voltages on all pins look normal.

[Jazz P Bass]

You may be seeing 'positive feedback'.

Try lifting R33 in the feedback path.
If the signal then is reduced at the output, reverse the OT primaries.

[mhuss]

Thanks for the suggestion, I just tried that, no difference (ignoring the extra gain ). I would expect if the primary was reversed it'd be howling.

[g1]

It doesn't have those funny brown sandbox 'protection' devices across the OT primaries does it?

[mhuss]

No, just power tube pin 3 connected directly to the primary (x2).

[alexradium]

i would check the OT out of circuit for mismatched wiring

[Helmholtz]

What are plate and screen voltages both measured directly at the sockets?

[xtian]

And check the grid and screen resistors, if you haven't.

[Helmholtz]

Resistor problems will show from actual voltages.

The observed signal asymmetry could be caused by:

- a non-conducting power tube (including open connection to one side of the OT primary)
- an open screen resistor.

[mhuss]

OK, I have officially lost it. Alex and Helmholtz are both pointing in the right direction.

The primary connections are blue, red, brown. With the primary disconnected:
bu-bn = 54 ohms
bn-rd = 54 ohms
rd-bu = 780 ohms!!!
How is this even possible? Has Ohm's law been repealed?
I measured and remeasured several times, both polarities. Same result. What the heck is going on?

EDIT: When all else fails, read the directions. On this particular transformer, red is the end and brown is center. So much for "following the original color code as much as possible."

I connected it with brown to B+ and red to the tube and now it's working OK.

I still don't get those resistance measurements, though...

[Chuck H]

EDIT: When all else fails, read the directions. On this particular transformer, red is the end and brown is center. So much for "following the original color code as much as possible."

Why would they do that!?!

Ok, but they put it in the literature. But why would they do that!?! What could possibly have been wrong with "red = B+"???

[mhuss]

Eh, probably some junior engineer with no history/experience. They let go of the old guy because he was costing too much.

[The Dude]

Agreed. Yes it's in the lit, but what about convention? I don't want them wiring my house........ special notes ("green is hot") in the breaker panel and all.

[Chuck H]

Agreed. Yes it's in the lit, but what about convention? I don't want them wiring my house........ special notes ("green is hot") in the breaker panel and all.

..."Except where we haven't rewired. We don't have complete notes on where that is"...

[mhuss]

For future reference, the Red and Brown are swapped and Blue stays where it is (for proper phase with the NFB) on a Hammond 1750G. Other variants (like the 1750RA) have the colors correct.